Functional recovery following injury to the central nervous system (CNS) of adult higher vertebrates is exceptionally limited, resulting in persistent neurological deficits such as loss of limb movement and sensation. As yet, there is a lack of an effective therapy to treat humans with CNS injuries such as spinal cord injury (SCI) and brain cortical injury. Although adult CNS neurons generally survive axotomy, axonal regeneration is transitory and only occurs over a confined area, hence retarding the re-formation of functionally-relevant synaptic contacts. Furthermore, the plastic capacity of the adult CNS is also restricted, thus hindering the re-organisation of uninjured pathways to functionally compensate for those ablated by the injury. Paradoxically, axotomised axons in the peripheral nervous system (PNS) have a high capacity to regenerate over long distances and frequently establish functionally-meaningful connections (Schwab (2004) Curr Opin Neurobiol 14, 118-124). This restriction in axonal regeneration/plasticity is in part due to the expression on myelinating oligodendrocytes of several proteins that have been shown to be potent inhibitors of neurite outgrowth, namely Nogo-A (Chen et al. (2000) Nature 403, 434-439; GrandPre et al. (2000) Nature 403, 439-444; Prinjha et al. (2000) Nature 403, 383-384), myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp) (McKerracher et al. (1994) Neuron 13, 805-811; Wang et al. (2002) Nature 417:941-944) (FIG. 1A).
Nogo-A contains multiple neurite outgrowth inhibitory domains exposed on the surface of oligodendrocytes: two are located within the amino-terminal region (amino-Nogo-A) and one in the C-terminal region (Nogo-66) (Oertle et al. (2003) J Neurosci 23, 5393-5406). Nogo-66 binds and signals through a glycosyl-phosphatidylinositol (GPI)-anchored leucine-rich repeat (LRR)-containing receptor on the neuronal surface known as the Nogo-66 receptor (NgR) (Fournier et al. (2001) Nature 409, 341-346). Although structurally unrelated, MAG and OMgp also bind and signal through NgR (Domeniconi et al. (2002) Neuron 35, 283-290; Liu et al. (2002) Science 297, 1190-1193; Wang et al. (2002) Nature 417:941-944). Signaling through NgR leads to the activation of the small GTPase RhoA which in turn activates Rho-associated kinase (ROCK) leading to a rigidification of the actin cytoskeleton and inhibition of axonal extension (Niederöst et al. (2002) J Neurosci 22, 10368-10376; Schweigreiter et al. (2004) Mol Cell Neurosci 27:163-174). All three ligands bind within the LRR region of NgR and have partially over-lapping binding sites (Fournier et al. (2002) J Neurosci 22, 8876-8883; Liu et al. (2002) Science 297, 1190-1193; Wang et al. (2002) Nature 417:941-944; Barton et al. (2003) EMBO J 22, 3291-3302). The receptor(s) for the inhibitory domains within amino-Nogo-A are unknown but have been shown to be distinct from NgR (Schweigreiter et al. (2004) Mol Cell Neurosci 27:163-174). MAG has also been found to signal through a close homologue of NgR known as NgR2 (Pignot et al. (2003) J Neurochem 85, 717-728; Venkatesh et al. (2005) J Neurosci 25, 808-822).
As NgR lacks a cytoplasmic domain, it utilizes several transmembrane proteins for signal transduction, namely the low affinity neurotrophin receptor p75NTR, TROY (a.k.a. TAJ) and LINGO-1 (LRR and Ig domain-containing, Nogo receptor-interacting protein a.k.a LRRN6A or LERN1) (Wang et al. (2002) Nature 420, 74-78; Carim-Todd et al. (2003) Eur J Neurosci 18, 3167-3182; Mi et al. (2004) Nat Neurosci 7, 221-228; Park et al. (2005) Neuron 45:345-351; Shao et al. (2005) Neuron 45, 353-359). TROY and p75NTR can functionally replace each other in the NgR receptor complex, whereas the presence of LINGO-1 is an absolute prerequisite for signaling to occur. The NgR receptor complex is therefore seen as a ternary complex comprising NgR as the ligand binding subunit and LINGO-1 as the common signal transducing subunit acting in concert with either p75NTR or TROY.
LINGO-1 is a single transmembrane protein expressed exclusively within the CNS predominantly on neurons and oligodendrocytes. The expression of LINGO-1 peaks in the early postnatal period and is up-regulated in the adult spinal cord upon injury. The ectodomain of LINGO-1 contains twelve tandem LRRs flanked by N- and C-terminal subdomains followed by a basic region and an Ig domain (FIG. 1B). Given that an AP fusion of the LINGO-1 ectodomain bound to COS-7 cells expressing NgR or p75NTR or both and, similarly, LINGO-1 co-precipitated with NgR or p75NTR in cells expressing all three proteins, LINGO-1 most likely forms a ternary complex with NgR and p75NTR by interacting with both simultaneously.
In addition to being expressed on neurons, LINGO-1 is also expressed in oligodendrocytes in the adult CNS (Mi et al. (2005) Nat Neurosci 8, 745-751). Inhibiting LINGO -1 signaling in oligodendrocyte cultures by either treatment with LINGO-1-Fc, down-regulation of the protein with RNAi or over-expression of DN-LINGO-1 augmented the differentiation of OPCs to myelinating oligodendrocytes. Furthermore, genetic ablation of LINGO-1 in mice increased the number of mature oligodendrocytes and, correspondingly, myelinated axons in the spinal cord. Inhibition of LINGO-1 signaling reduced the activation of RhoA and increased the activity of Fyn kinase, both of which are reported to promote oligodendrocyte differentiation, although the actual ligands/interactions responsible for activating LINGO-1 signaling have yet to be exemplified. This has led to the conclusion the LINGO-1 is a negative regulator of myelination.
Multiple Sclerosis (MS) is a chronic inflammatory disease of the CNS characterised by demyelination and axonal degeneration leading to multiple neurological deficits. Although remyelination of axons can occur early in the disease, at some point remyelination fails completely leading to accelerated axonal degeneration and irreversible damage. Remyelination most likely arises from the differentiation of adult oligodendrocyte precursor cells (OPCs) which migrate to the margins of active lesions. As LINGO-1 negatively regulates myelination, blockade of LINGO-1 may augment remyelination, attenuate axonal degeneration, promote axonal regeneration and thus attenuate, halt or even reverse the progress of demyelinating diseases such as MS.
Blockade of LINGO-1 has also been shown to improve the survival of dopaminergic neurons and reduce behavioural abnormalities in rodent models of Parkinson's disease (Inoue et al. (2007) Proc Natl Acad Sci USA 104, 14430-14435).